Metal oxides are used in a wide range of applications, such as in:                solid oxide fuel cells (in the cathode, anode, electrolyte and interconnect);        catalytic materials (automobile exhausts, emission control, chemical synthesis, oil refinery, waste management);        magnetic materials;        superconducting ceramics;        optoelectric materials;        sensors (eg. gas sensors, fuel control for engines);        structural ceramics (eg. artificial joints).        
Conventional metal oxides typically have grain sizes that fall within the micrometer range and often are supplied in the form of particles having particle sizes greater than the micrometer range. It is believed that metal oxides that are comprised of nanometer sized grains will have important advantages over conventional metal oxides. These advantages include lower sintering temperatures, potentially very high surface areas, and sometimes improved or unusual physical properties. However, the ability to economically produce useful metal oxide materials with nanometer-sized grains has proven to be a major challenge to materials science. It has proven to be difficult to make such fine-scale metal oxides, particularly multi-component (complex) metal oxides, with:                a) the correct chemical composition;        b) a uniform distribution of different atomic species;        c) the correct crystal structure; and        d) a low cost.        
Many important metal oxides have not yet been produced with very fine grains, especially multi-component metal oxides. This is because as the number of different elements in an oxide increases, it becomes more difficult to uniformly disperse the different elements at the ultra-fine scales required for nanometer-sized grains. A literature search conducted by the present inventors has shown that very small grain sizes (less than 20 nm) have only been attained for a limited number of metal oxides. The reported processes used to achieve fine grain size are very expensive, have low yields and can be difficult to scale up. Many of the fine-gained materials that have been produced do not display particularly high surface areas, indicating poor packing of grains.
At this stage, it will be realised that particles of material are typically agglomerates of a number of grains. Each grain may be thought of as a region of distinct crystallinity joined to other grains. The grains may have grain boundaries that are adjacent to other grain boundaries. Alternatively, some of the grains may be surrounded by and agglomerated with other grains by regions having a different composition (for example, a metal, alloy or amorphous material) to the grains.
Methods described in the prior art for synthesising nano materials includes gas phase synthesis, ball milling, co-precipitation, sol gel, and micro emulsion methods. The methods are typically applicable to different groups of materials, such as metals, alloys, intermetallics, oxides and nonoxides.
In our international patent application no. PCT/AU01/01510 (WO 02/42201), we describe a method for producing metal oxide particles having nano-sized grains. This method comprises the steps of:
a) preparing a solution containing one or more metal cations;
b) mixing the solution from step (a) with one or more surfactants under conditions such that surfactant micelles are formed within the solution to thereby form a micellar liquid; and
c) heating the micellar liquid from step (b) to form metal oxide, the heating step being undertaken at a temperature and for a period of time to remove the surfactant and thereby form metal oxide particles.
The metal oxide particles formed by this process have a disordered pore structure. The entire contents of our international patent application no. PCT/AU01/01510 (WO 02/42201) are expressly incorporated herein by cross reference.
The process described in our international patent application no. PCT/AU01/01510 provides a method that allows for economic production of metal oxide particles. The method is particularly useful for preparing complex metal oxide particles which contain two or more metals. Throughout this specification, the term “complex metal oxide” will be used to describe a metal oxide having two or more metals therein. The complex metal oxide particles prepared by this method have a very homogenous distribution of the metal species throughout the particles. A uniform crystal structure is also obtained, which provides further indication of the homogeneity of the mixed metal oxide product. Furthermore, the process is easily able to be scaled up.
In the process described in our earlier international patent application no. PCT/AU01/01510, step (a) involved preparing a solution containing cations of all of the metals to be incorporated into the complex metal oxide. It was believed that putting all of the metals into solution in assisted in obtaining a homogenous distribution of the metals in the mixed metal oxides produced by the process.
U.S. Pat. No. 6,139,816 (Liu et al), the entire contents of which are here incorporated by cross reference, describes a process for the production of metal oxide powders, wherein metal oxide precipitates or metal oxide gels are formed by mixing surfactant with aqueous solutions containing metal salts. The surfactant and salt types are chosen so that a precipitate or gel of the metal oxide forms on mixing. The metal oxide precipitates or metal oxide gels are separated from the rest of the mixture and then further heat treated to obtain metal oxide powders.
U.S. Pat. No. 5,698,483 (Ong et al), the entire contents of which are herein incorporated by cross reference, describes a process for producing nano-size powders. Ong et al mixes a solution containing metal cations with hydrophilic polymers to form a hydrophilic polymer gel. The hydrophilic polymer gel is then heated to drive off water and organics, leaving a nanometer-sized metal oxide powder.
U.S. Pat. No. 6,328,947 (Monden et al), the entire contents of which are herein incorporated by cross reference, describes a process for producing fine particles of metal oxide having diameters of about 20 nm or smaller by hydrolyzing metal halides in the presence of an organic solvent. In Monden et al, metal oxides are formed by hydrolysis of metal halides in organic solution. The metal oxide precipitates are then separated from the mother solution (for example, by filtration, centrifugation and so forth), washed and then dried.
U.S. Pat. No. 5,879,715 (Higgins et al) and U.S. Pat. No. 5,770,172 (Linehan et al), the entire contents of which are herein incorporated by cross reference, describe processes for production of nano-particles by using microemulsion methods. In these processes, a microemulsion is formed and metal oxides are precipitated within the microemulsion micelles, thereby limiting the size of the metal oxide particles to approximately the size of the droplets. In Higgins et al, two water-in-oil emulsions are prepared, one with dissolved metal salt in the water droplets and the other with a reactant in the water droplets. The microemulsions are mixed and when the reactant-containing droplets contact the metal solution-containing droplets, precipitation of metal oxide occurs. In Linehan et al, a water-in-oil microemulsion is formed with dissolved metal salt in the water droplets. A reactant is then added to the system, for example, by bubbling a gaseous reactant therethrough to precipitate metal oxide in the water droplets.
U.S. Pat. No. 5,788,950 (Imamura et al), the entire contents of which are herein incorporated by cross reference, describes a process to synthesise complex metal oxide powders using liquid absorbent resin gels. In Imamura et al, a solution conning at least two dissolved metals is contacted with a liquid absorbent resin such that at least two metals are present in the liquid absorbent resin after combining with the solution. The liquid absorbent resin is allowed to swell and gel. The swollen gel is treated by changing at least one of the pH or temperate of the swollen gel to form a precursor material. The precursor material is pyrolyzed and calcined to form the mixed metal oxide powder.
DE 19852547, the entire contents of which are herein incorporated by cross reference, describes a process for producing metal oxide powders by treating aqueous solutions of metal salts with an aqueous base to produce a precipitate (condensate) in the presence of a water soluble stabiliser.
United States patent application number 2005/0008777 (McCleskey et al), the entire contents of which are herein incorporated by cross reference, describes a process for forming metal oxide films. The process involves preparing solutions of one or more metal precursors and soluble polymers having binding properties for the one or more metal precursors. After a coating operation, the resultant coating is heated at high temperatures to yield metal oxide film.
All of the above described United States patents and patent application and DE 19852547 rely upon the formation of solutions containing all of the metal species that become incorporated onto the metal oxide material.
Production of appropriate solutions, in practice, can occasionally prove to be problematical. For some metals, the range of salts available is very limited or even non-existent. Mixing of different salt types, e.g. chlorides and nitrates, can lead to problems with unwanted precipitation of metal elements. Control of the solution chemistry could be very important to ensure that all of the metal compounds supplied as raw materials to the process went into solution and precipitation of other metal compounds from the solution was avoided. In addition, the presence of large amounts of counter-ions (eg NO3) can lead to problems in controlling heat treatment and unwanted precipitation of other elements in the solution.